Many years ago I milled a small flat into the threads on the nose and drilled the back plate to fit a grub screw Now I don't know if this was a good idea or not but it worked and didn't seem to effect the accuracy when a normal chuck was refitted.

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People say you only live once ! I say thank F@*K can't afford to do it twice.

The chuck is located on the plain register, so provided it's not a sloppy fit then I suppose a flat on the thread won't matter. It depends on the lathe as to how you can achieve this. On the Boxford, for example, it's possible to reduce the thickness of the spindle bearing cover plate to expose a plain section of the spindle. You can then fix a split collar to the back of the chuck back plate which then clamps around the spindle. There was mention of this on the Boxford Users' Group recently.

We had a Simonet instrument lathe at work and all the chucks and faceplate had a loose split collar which applied a bit of squeeze of the backplate onto the register. You couldnt budge them without undoing the collar.